Electronic systems especially those for communications applications operated at radio frequencies (RF) require small bandpass filters and oscillators. The oscillators are for generation of frequency signals and the bandpass filters are for selection of (transmit or receive) signals within certain bandwidth (BW) at a given frequency. Some examples of the systems include global positioning systems (GPS); mobile telecommunication systems: Global Systems for Mobile Communications (GSM), personal communication service (PCS), Universal Mobile Telecommunications System (UMTS), Long Term Evolution Technology (LTE); data transfer units: Bluetooth, Wireless Local Area Network (WLAN); satellite broadcasting and future traffic control communications. They also include other high frequency systems for air and space vehicles.
Bandpass filters for RF signals are fabricated using different technologies: (a) ceramic filters based on dielectric resonators, (b) filters using surface acoustic wave resonators (SAW), and (c) filters using thin film bulk acoustic wave resonators (FBAR). Both SAW and FBAR are used when dimensions of the systems are limited. Currently, SAW devices are mainly used in volume applications at frequencies below 2 GHz whereas FBARs are dominant in systems operated at frequencies from 2 to 4 GHz. Due to large volumes, SAW or FBAR RF filters in handsets are manufactured by microelectronic fabrication processes on wafers using piezoelectric materials such as LiNbO3 (for SAWs) and AlN (for FBARs).
Surface Acoustic Wave (SAW) Filters
The development of SAW devices dated back to 1965, when the first SAW devices were made. Earlier research work in SAW devices was mainly to fulfill the needs of radar signal processing. In the 1980s and 1990s, the main development efforts were focused on low loss filters particularly for mobile phones. The basic principles of SAW devices can be understood by considering a basic SAW structure. FIG. 1 shows a schematic diagram of a prior art surface acoustic wave filter (100) on a piezoelectric substrate (110), with an input inter digital transducer IDT1 (120) with a center-to-center distance between adjacent electrodes controlled to a “pitch” and connected to an electrical signal source (130) to excite acoustic waves (140) with a velocity v and at a frequency fo=v/(2×pitch), an output inter digital transducer IDT2 (150) with a center-to-center distance between adjacent electrodes again also controlled to the “pitch” to receive the acoustic waves (140) and covert them into an output electrical signal (160). Electrical signals in the signal source (130) at frequencies other than fo cannot excite resonant acoustic waves with sufficient level to reach the output IDT2 (150) to generate an output in the output terminals. Once a SAW filter is fabricated, the central frequency fo of transmission and bandwidth BW are fixed by the geometry of the filter and by materials used. The only electrical signals that are allowed to reach the output inter digital transducer from the input inter digital transducer are those with a frequency to be within the bandwidth of a center frequency fo.
The main properties of piezoelectric materials for filters are propagation velocity of acoustic waves, electrode pitch and coupling coefficients, where the velocity of acoustic waves and the electrode pitch determine the resonant frequency and the coupling coefficients affect the band width. Velocities values for several piezoelectric substrates are: LiNbO3˜4,000 m/s, ZnO˜6,300 m/s, AlN˜10,400 m/s and GaN˜7,900 m/s. As an example, to obtain a filter on LiNbO3 with a central frequency fo=2 GHz, the wavelength of the acoustic wave is λ=(4000 msec)/(2×109/sec)=2×10−4 cm. Therefore, the value of electrode pitch in FIG. 1 is then equal to (½)λ or 1 μm. Assuming that the width of electrodes and the space between adjacent electrodes are equal, the electrode width is then 0.5 μm.
Tunable Filters
For each communication band, there are two frequencies close to each other: one for transmitting and the other for receiving. For mobile communications, there are about 40 bands. More bands are expected for the next generation long term extension technology. Table 1 gives several selected bands for mobile communications used in different regions or countries. In each band, there is a transmit band or Tx Band at a transmit band central frequency foTR with a transmit bandwidth BWTR. There is also an associated receive band or Rx Band at a receive band central frequency foRE with a receive bandwidth BWRE. The separation between the transmit band and the receive band is given by: foRE−foTR.
Table 1 Band frequencies and bandwidth for some of the bands assigned to mobile handsets and base stations.
TABLE 1Band frequencies and bandwidth for some of the bands assigned to mobile handsets andbase stations.BandfoTR (MHz)BWTR (MHz)foRE (MHz)BWRE (MHz)foRE-foTR (MHz)(foRE-foTR)/foTRRegion11920-1980602110-2170601909.8%Asia, EMEA, Japan21850-1910601930-199060804.3%N. America, Latin Am.31710-1785751805-188075955.4%Asia, EMEA41710-1755452110-215545400 23%N. America, Latin Am.5824-84925869-89425455.4%N. America, Latin Am.72500-2570702620-2690701204.7%Asia, EMEA8880-91535925-96035455.0%EMEA, Latin Am.12699-71617729-74617304.2%N. America
Due to the large number of bands used in the mobile handsets in different regions and countries, and even in the same country, a practical handset needs to have an RF front end covering several frequency bands. A true world phone will need to have about 40 bands, each with a transmit band and receive band. As each RF filter has only one central frequency of resonant and a bandwidth which are fixed, therefore, such a true world phone will need to have 80 filters for the front end. Due to the resource limitations, some designers design mobile phone handsets to cover 5 to 10 bands for selected regions or countries. Even with this reduced number of bands, the number of RF filters currently required is still large (10 to 20 units). Therefore, there is a strong need to reduce the dimensions/cost of the RF filters and to reduce the number of filters for the same number of operation bands by using tunable RF filters, each to cover at least two frequency bands, so that the number of filters can be reduced in the mobile handsets and many other microwave and wireless systems. Thus, it would be ideal to develop an RF filter which can cover as many bands or frequency ranges as possible so that the size and power consumption of RF front ends in a mobile phone handset and microwave systems can be reduced. In Table 1, values of (foRE−foTR)/foTR are listed. It is seen that majority has a value of 10% or less: mostly ˜5%. Therefore, tunable filters with a tuning range of 10% or more will be highly valuable for communications.
In order to fulfill the demands for RF filters covering as many bands or frequency ranges as possible, tunable SAW inter digital transducers and reflectors have been invented and disclosed in U.S. patent application Ser. No. 14/756,554 by the inventors of the present application. This invention provides tunable surface acoustic wave resonators utilizing semiconducting piezoelectric layers having embedded or elevated electrode doped regions. Both metallization ratio and loading mass are changed by varying a DC biasing voltage to effect a change in the resonant frequency. A plurality of the present tunable SAW devices may be connected into a tunable and selectable microwave filter for selecting and adjusting of the bandpass frequency or an tunable oscillator by varying the DC biasing voltages.
Tunable SAW IDTs and Filters
FIG. 2 shows a schematic top view of a tunable surface acoustic wave (SAW) filter (200a) disclosed in U.S. patent application Ser. No. 14/756,554 by the inventors of the present application. The SAW filter (200a) has an input inter digital transducer IDT1 (220) and an output inter digital transducer IDT2 (250). The IDT1 and the IDT2 are made on a first piezoelectric layer (210) deposited on a support substrate (210S). The first piezoelectric layer (210) in the SAW structures is selected from a group of piezoelectric materials including: LiNbO3, LiTaO3, ZnO, AlN, GaN, AlGaN, LiTaO3, GaAs, AlGaAs and etc.
The IDT1 (220) comprises an input positive electrode pad (220PM) on an input positive electrode pad doped region (220DP); an input negative electrode pad (220NM) on an input negative electrode pad doped region (220DN); input positive electrode fingers (220P-1, 220P-2, 220P-3) each on one of the respective input positive electrode doped regions (DP-1, DP-2, DP-3); input negative electrode fingers (220N-1, 220N-2, 220N-3) each on one of the respective input negative electrode doped regions (DN-1, DN-2, DN-3). The input positive electrode doped regions (DP-1, DP-2, DP-3) and input negative electrode doped regions (DN-1, DN-2, DN-3) are doped piezoelectric semiconductors. A center-to-center distance between adjacent input positive electrode fingers and input negative electrode fingers (or between the input positive electrode doped regions and the input negative electrode doped regions) is controlled to a “pitch” b. The input electrode fingers are connected to an electrical signal source (230) to excite surface acoustic waves (240) at a frequency f˜v/(2×b), v being the velocity of the surface acoustic waves. Similarly, the output inter digital transducer IDT2 (250) comprises an output positive electrode pad (250PM) on an output positive electrode pad doped region (250DP); an output negative electrode pad (250NM) on an output negative electrode pad doped region (250DN); output positive electrode forgers (250P-1, 250P-2, 250P-3) each on one of the respective input positive electrode doped regions (DP-1′, DP-2′, DP-3′); output negative electrode fingers (250N-1, 250N-2, 250N-3) each on one of the respective output negative electrode doped regions (DN-1′, DN-2′, DN-3′). The output positive electrode doped regions (DP-1′, DP-2′, DP-3′) and output negative electrode doped regions (DN-1′, DN-2′, DN-3′) are doped piezoelectric semiconductors. A center-to-center distance between adjacent output positive electrode fingers and output negative electrode fingers (or between adjacent output positive electrode doped regions and output negative electrode doped regions) is controlled to the “pitch” b′ which is preferably the same as b to receive the surface acoustic waves (240) and covert them into an output electrical signal Vout across an output resistor R (260).
The input inter digital transducer (220) and output inter digital transducer (250) are kept apart by an IDT center-to center distance (200D). The Input electrode doped region widths (a) are kept to be substantially equal to half of the pitch (b) so that spacing between adjacent input electrode doped regions (c) is also substantially equal to half of the pitch (b). Similarly, the output electrode doped region width (a′=a) is kept to be substantially equal to half of the pitch (b′=b) so that spacing between adjacent output electrode doped regions (c′) is also substantially equal to half of the pitch (b′=b). The input electrode finger width (m) is selected to be the same as the output electrode finger width (m′) and the finger widths (m, m′) are no more than electrode doped region widths (a, a′).
An input DC biasing voltage VDC is connected to the input inter digital transducer IDT1 through blocking inductors LN-1 and LP-1 to tune and adjust the frequency of the surface acoustic waves to be excited by IDT1 whereas an output DC biasing voltage V′DC is connected to the output inter digital transducer through blocking inductors LN-1′ and LP-1′ to tune and adjust frequency of the surface acoustic waves to be received or detected by IDT2. Value of the input DC biasing voltage VDC is preferably selected to be same as the output DC biasing voltage V′DC to achieve synchronous tuning and adjustment for the frequencies. The value of pitch (b, b′) is selected during the design and fabrication of the SAW device and the wavelength of surface acoustic waves to be excited and to propagate is: λ=2b. The value of λ together with the velocity v of the surface acoustic waves thus determine a unique central frequency f=v/λ of the excitation, propagating and detection of surface acoustic waves. The tuning of frequency is based on the adjustment of mass loading (ML) and metallization ratio (MR) associated with the electrode doped regions and electrode fingers which can be found in extensive detail in U.S. patent application Ser. No. 14/756,554.
In this earlier invention, as described above, the DC biasing voltages are provided through blocking inductors (LN-1, LP-1, LN-1′, LP-1′) to the input and output IDTs, wherein the blocking inductors are to separate the DC bias and the RF signals. While thin film inductors (LN-1, LP-1, LN-1′, LP-1′) are effective in isolating RF signals from the DC biasing circuit, thin film inductors have two disadvantages. Thin film inductors are usually made as metal coils which occupy fairly large areas and thin film coils are relatively difficult to fabricate and therefore are not practical. Therefore, tunable SAW IDTs with an improved RF isolation method are needed for practical tunable SAW resonator, filters or oscillators.